ISX ELECTRONIC INJECTORS QUESTION

[DTidwell05]

Hi, I am new here so go easy on me. I recently had a truck tune changed by a local shop because it was making boost at idle. When it came back they told me I need to replace injector # 3 because its hanging open. The injectors are all new from {aftergarbage diesel}. What is weird to me is that the truck sounds great until it reaches about 140-150 degrees coolant temp then it starts a fuel knock. The mechanic shop is telling me they can turn off # 3 injector with the computer and the knock goes away. I completely understand that makes since, but I just don't understand why it would only do it once the engine warms up. Its a night and day difference in the sound of the engine once warm and it will start a little white smoke out of the exhaust and smells super rich. I have a feeling its tune related because the new injector didn't do that a week ago before going to the shop.
2013 Peterbilt 367 with ISX

Thanks for any advice.
Thanks, Dustin